They had planned and trained for it for five days last year, playing out the disastrous scenarios of a hypothetical Hurricane Pam. But when the real disaster stuck, they appeared to be paralyzed.

President Bush on Tuesday acknowledged "serious problems" in the government's response to emergencies, and accepted responsibility for the federal government's failures in responding to the disaster.

"Katrina exposed serious problems in our response capability at all levels of government and to the extent the federal government didn't fully do its job right, I take responsibility," Bush said during a news conference.

There are plenty of unanswered questions about what went wrong, when it went wrong and who is at fault.

In the hurricane's aftermath, thousands of people trapped in the submerged city began asking how they got left behind without food and water. And why?

Why did it take so long to get help to stranded people? Where were the helicopters to drop food and emergency supplies? And eventually, why were people who sought safety in shelters still without food and water five days after the storm?

In the aftermath, the questions grew sharper: Why did aerial shots of the flooded city show hundreds of school and city buses window-deep in water? Why hadn't anyone used those buses to move people out? Did Amtrak really offer residents seats on trains the company moved out of harm's way? And if so, who refused that offer and why?

People also asked why FEMA wouldn't allow the delivery of 20,000 trailers Sen. Trent Lott found. Lott, a Republican from Mississippi, lost his own home.

Then there's perhaps the most alarming question of all: Is the Department of Homeland Security too big a bureaucracy to be effective in its mission?

"We had our first post-9/11 task and we've miserably failed," said former U.S. Rep. Tim Roemer, an Indiana Democrat who was a member of the 9/11 Commission.

"Our government couldn't drop water to our most needy citizens," Roemer said. "We couldn't get generators to people in hospitals. We didn't go by any evacuation plan."

Plenty of blame

In addition to Brown, other public officials face criticism and hard questions about what they did and didn't do. Chief among them are Michael Chertoff, who heads the Department of Homeland Security, Louisiana Gov. Kathleen Blanco and New Orleans Mayor Ray Nagin.

Chertoff has insisted for two weeks he had no warning of how bad Katrina could be.

But the National Weather Service issued a detailed message a day before Katrina made landfall as a Category 4 hurricane, saying buildings would be leveled, high-rises crippled and most of the area will be uninhabitable for weeks, perhaps longer.

Chertoff, whose department oversees FEMA, had continued to downplay the significance of the levee breaks in New Orleans, even as floodwaters consumed 80 percent of the city.

Blanco is under fire over whether she asked the right people in Washington for help soon enough. She has been accused of engaging in a bureaucratic turf war that delayed the National Guard response as New Orleans spiraled into anarchy.

Help turned away?

State officials also are being blamed for turning back assistance during the critical first few days. Sheriff Steve Simpson, of Loudon County, Virginia, sent 22 deputies with supplies and 14 vehicles, including four all-terrain vehicles. But he called them back when Louisiana state police officials waved him off.

"I said, 'What if we just show up?' and he says, 'You probably won't get in," Simpson told CNN. Later that night, Blanco cleared legal hurdles that would have allowed local officials to accept the help, but no one ever got back to Simpson.

"I'm very frustrated, trying to figure out what went wrong in that process," Simpson said.

The White House has suggested that Gov. Blanco also failed to call early enough for the federal help she needed. The governor's office says that before, during and after the storm, Blanco's message to the president was consistent. (Watch the video on political defensive moves -- 1:56)

Blanco lashed out at FEMA Tuesday for what she said was a "lack of urgency and lack of respect" involving the recovery of bodies of Hurricane Katrina victims.

Blanco said she ordered the state to sign a contract with Kenyon International Monday after Chertoff failed to live up to renew the private disaster recovery firm's contract. The company has been recovering bodies in New Orleans.

Kenyon worked for the Australian government to identify the remains of tourists killed during the December tsunami, and the company handled the remains of plane passengers who crashed into a Pennsylvania field during the September 11 attacks.

Kenyon told the state that if they didn't get a contract soon, they would be forced to leave as soon as they professionally could.

"In death, as in life, our people deserve more respect than they have received," Blanco said.

Empty train

Nagin, whose desperate plea for help in the days after the storm made him a folk hero to some, faces criticism for turning away resources that could have moved more people out of the city faster.

The mayor's disaster plan called for mobilizing buses and evacuating the poor, but he did not get it done. He said he could not find drivers, but Amtrak says it offered help and was turned down, so a train with 900 seats rolled away empty a day and a half before the storm. (Watch the video detailing the failed evacuation plan -- 2:11)

"One of the problems that we're facing at the federal level and at the state level and at the local level -- and again, not casting blame anywhere, is a total systemwide failure, because people making decisions hesitated," Senate Majority Leader Bill Frist, a Republican from Tennessee, told CNN.

Louisiana Sen. David Vitter, a Republican, said he initially was impressed by how quickly federal authorities mobilized before the storm. But after it hit, nothing happened for days.

"There was absolutely no execution," Vitter told CNN.

"I was very happy with how quickly the president had signed his first emergency order," he said. "The FEMA director was on the ground before the storm. FEMA teams were on the ground. But then Monday, Tuesday, Wednesday, absolutely no execution. I don't know what they were doing."

Bureaucratic breakdown

The accusations and the public outrage make federal, state and local leaders jittery and defensive. They know that just a few days ago Brown's job appeared to be safe.

Vitter believes the time will come soon enough to answer the hard questions.

"I don't have a doubt in the world that all of these questions are going to be asked in a very forceful, focused way," he said. "So there are a lot of folks, myself included, just as a citizen of Louisiana, who are going to demand straight answers and get the full story, wherever that leads."

He said that the blame does not rest solely with Brown.

"This wasn't a failure of one person, although it was that also," Vitter told CNN. "It was a failure of the whole bureaucracy, and the solution to that isn't getting a new head bureaucrat or a new type of head bureaucrat. I think the whole bureaucratic FEMA model is what has to be probably discarded. "